1. Field of the Invention
This invention relates generally to equalizers that compensate for impairments introduced by interleaved devices, including for example interleaved analog-to-digital converters (ADC).
2. Description of the Related Art
Time interleaving can be used to increase the effective speed of a device. A non-interleaved device contains a single signal path. An interleaved device contains multiple parallel signal paths (or channels) and signals are time sequentially routed to the different channels. Thus, the effective speed of the device is the aggregate speed of the channels. One example of an interleaved device is an interleaved analog-to-digital converter (ADC). For example, rather than using a single ADC path that operates at 10 Gb/s, eight slower ADC channels that operate at 1.25 Gb/s each can be time interleaved to achieve an effective rate of 10 Gb/s for the interleaved ADC.
Interleaved ADCs are beginning to be applied in high speed communication systems. In these systems, digital signal processing may be used to compensate impairments of the communications channel. Electronic dispersion compensation (EDC) receivers for fiber optic communications at 10 Gb/s and beyond are an example of such systems. Of current interest is the emerging IEEE 802.3aq standard for 10 Gb/s Ethernet over multimode fiber links, which is enabled by EDC technology. Multimode fibers are typically used in local area network (LAN) applications, in links whose length could reach or exceed 300 meters. A large percentage of the existing fiber population consists of legacy fibers that exhibit large multimode dispersion. Interest in EDC for multimode fibers has grown significantly in recent years as a result of the standardization activities of the IEEE 802.3aq Task Force, which is developing the 10GBASE-LRM standard for 10 Gb/s Ethernet over multimode fiber based on the application of EDC technology. This standard is in a draft state, currently documented in IEEE Draft P802.3aq/D4.0, Draft amendment to: IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Amendment: Physical Layer and Management Parameters for 10 Gb/s Operation, Type 10GBASE-LRM, referred to herein as IEEE 802.3aq/D4.0 or 10GBASE-LRM or simply LRM, and incorporated by reference.
The 10 Gb/s data rate in these systems can make interleaved ADCs attractive compared to non-interleaved ADCs. However, it is well known that ADC interleaving techniques are sensitive to mismatches of the offset, gain, and sampling phase among the different channels. These impairments are collectively known as fixed-pattern noise. A less well known form of fixed-pattern noise results from mismatches of the frequency responses of the time interleaved track-and-hold units that typically are located towards the front of each ADC channel.
Communication systems may already use some form of equalization in the receiver, for example to correct for dispersion or other effects. However, these equalizers typically treat the ADC as a black box and do not distinguish between interleaved ADCs and non-interleaved ADCs. To the equalizer, the interleaved ADC is treated effectively as a single path high-speed ADC. The equalizer may compensate for some of the impairments introduced by the ADC. However, since the equalizer does not account for the interleaved nature of the ADC, it may not be able to correct for impairments that vary from channel to channel (i.e., channel-dependent impairments), such as mismatches in the frequency responses of the time interleaved track-and-hold units.
Thus, there is a need for approaches that can compensate for impairments (especially channel-dependent impairments) introduced by interleaved devices.